Isotropic conductive adhesives (ICAs) have been commercially available as an alternative to solder interconnects in electronic packaging for many years. An ICA typically consists of a non-conductive adhesive matrix filled with conductive particles. The mechanical strength of the ICA comes from the adhesive matrix, while the conductive fillers provide electrical conductivity.
The most common ICAs in the electronics industry consist of epoxy filled with silver flakes or particles. FIG. 1 is a schematic drawing showing an ICA of this type used to connect a component 2 to a substrate 4. Both of the component 2 and substrate 4 have an electrical contact 6. The ICA, which comprises a polymer resin matrix 8 and silver flakes 10 is used to electrically connect the two contacts 6. The silver flakes 10 form a conductive network that allows for conduction of electricity in all directions. Before the adhesive is cured, the silver flakes (in the adhesives) have a tendency to be oriented both during application and under the influence of gravity. This results in a better conductivity in the plane of the substrate compared to conductivity normal to this plane and hence the conductive properties of the adhesive are not truly isotropic. Traditionally, ICA compounds have been heavily loaded with silver particles, typically in the range of 25-30 volume % (up to 80 wt %), to ensure sufficient electrical conductivity also in the z-direction. This leads to a high metal content for the adhesive. Such a high metal particle loading causes significant changes in the mechanical properties of the adhesives, including increased bulk modulus, reduced flexibility (a more brittle response) and an excessive use of precious metal.
Metal coated glass beads have also been proposed as the filler in ICAs. However the use of glass beads creates a mismatch in thermal coefficient of the bead with the epoxy matrix and this results in degradation and loss of conductivity when the adhesive undergoes thermal cycling. In addition, the rigidity of the glass beads and difficulties in generating tightly controlled size distribution for the particles results in limited contact areas with the surfaces that are adhered together, further limiting conductivity.
U.S. Pat. No. 6,942,824 discloses an electrically conductive adhesive including a resin component, a photoinitiator, and metal-coated polymer beads. The resin is UV cured. The metal coating of the beads provides electrical conductivity. The use of polymer beads avoids some of the disadvantages associated with glass beads. The polymer beads are between 15 and 30 μm in diameter and are coated with gold or silver having a thickness of between 20 and 100 nm. The metal layer is a uniform coating formed by ‘polymer bead shell metal plating’, which is disclosed as comprising the formation of a uniform copper layer followed by plating with an additional layer, or by electroplating, which typically involves the use of a continuous nickel seed layer followed by a gold or silver layer. The use of nickel may not be desirable as it is classified as Carcinogenic 2B (possible carcinogen for humans) and is related to skin sensitisation and allergies.